One of the causes of tire noise is cavity resonance sound caused by vibration of air filled in the inside of a tire. When the tire is rotated under a load, a ground-contacting portion in a tread portion of the tire vibrates because of irregularities on a ground surface and this vibration causes vibration of the air inside the tire and generates the cavity resonance sound. This cavity resonance sound is generated by vibration of air inside the tire caused by vibration of a ground-contacting portion in a tread portion of the tire which is generated by irregularities on a ground surface when the tire is rotated under a load. In this cavity resonance sound, it has been known that frequencies perceived as noise are approximately 200 to 250 Hz although they differ by tire size. Therefore, for the purpose of reducing tire noise, it is important to reduce noise level in a range of the above frequencies.
As a method of reducing noise caused by the cavity resonance phenomenon as above, there has been a proposed approach where a sound-absorbing material is added to the inside of a tire to absorb resonance (refer to Patent Document 1, for example). However, a full consideration has not been given to characteristics of the sound-absorbing material in this approach, and it is not necessarily considered that the approach has achieved an excellent result in noise reduction. [Patent Document 1] Japanese patent application Kokai publication No. Sho62-216803